The science magazine of Imperial College

Is human memory unique?

What sets humans apart from other animals? Language? Creativity? Some think that the answer is actually memory, and that only humans have the ability to re-experience events as though we were there.

While no-one disputes that animals have memories for facts (semantic memory), it has been argued that they have limited or non-existent episodic memory – they are unable to undergo what the influential researcher Endel Tulving called ‘mental time travel’: the ability to relive experiences in your mind.

From observing your pet, you might think this is wrong, and some researchers agree with you. Some, for example Nicola Clayton’s group at Cambridge, have designed inventive experiments to question Tulving’s assumption of human uniqueness. The most compelling evidence for episodic-like memory in animals comes not from our closest relatives such as gorillas or chimpanzees, but from western scrub jays, a bird native to North America.

Tulving’s original criteria for episodic memory was the ability to know the ‘what’, ‘where’ and ‘when’ of an event. In 1998 Nicola Clayton and her colleagues used an ingenious experiment to test scrub jays’ ability to know what, where and when. They capitalised on the birds’ natural instinct to cache food (bury it to dig up later) and their natural preference for worms over peanuts.

In the experiment, the scrub jays were allowed to store either peanuts or worms on one side of a tray, and after 120 hours, they cached the other type of food on another side. Four hours later, they were allowed to retrieve whichever food they wanted. If the worms had been most recently stored, they were still fresh, but if they had been cached 124 hours ago, they had turned rotten. After learning this, the birds dug up peanuts if there had been an 124 hour delay between caching and retrieval, but worms if there had only been a short delay: they knew what (peanuts or worms) was where (which side of the tray) and when (how long ago it had been cached).

Importantly, this was not just a result of forgetting where the older cache was, as another group of scrub jays were taught that the worms never degraded, as the researchers replaced the old worms with fresh ones. This group continued to choose the worms after 124 hour delays. Therefore, the jays remembered how long ago the food had been cached and changed behaviour according to the information about what happened in that delay. This type of memory has also been found in rats and even cuttlefish. You could argue that this type of memory does not necessarily involve mental time travel. However, without talking to animals it’s probably as close as we can get to demonstrating episodic memory in animals.

However, mental time travel does not only mean you can mentally go backwards into the past, but means you can also project yourself into the future. While remembering the past and imagining the future might seem very different to most of us, they are probably two sides of the same coin: patients with amnesia who cannot remember their past also cannot imagine their own future.

So can animals project themselves into the future? This has also been tested in scrub jays: when overfed on peanuts before caching they tend to preferentially store kibble if it’s also available, as they have eaten too many peanuts at that point. However, if they are then trained that before digging up their horde they will be overfed on kibble, they can plan for this future. At caching, they will store more peanuts than kibble, even though they have just eaten lots of them. They know that in the future they will be fed up of kibble so will want peanuts: they dissociate their present and future states of hunger.

Another interesting experiment by the same research group showed a connection between the birds’ past state and their future plans. Here, the birds cached their food either with another scrub jay watching or alone. A few hours later they were allowed to move their food with no-one watching. Birds only moved the food (so no other bird knew where it was) if another bird was watching and if the caching bird had previous experience of stealing another bird’s caches. They seemed to relate their own past experience of stealing to a possible future of another bird stealing their food: an impressive ability.

While there is good evidence for animals knowing what, where and when something happened and that they can plan for the future, there is no way we can ever prove that animals truly undergo ‘mental time travel’. Perhaps our experience of the past and future is so tied up with language that animals could not have this ability without language anyway. But these experiments still tell us about the wonder of the animal world, and how humans may not be so unique after all.